The following table shows the peak errors (in units of epsilon) found on
various platforms with various floating point types. Unless otherwise specified
any floating point type that is narrower than the one shown will have
effectively zero error.

The implementation of these functions is very similar to that of beta, and is
based on combining similar power terms to improve accuracy and avoid spurious
overflow/underflow.

In addition there are optimisations for the situation where delta
is a small integer: in which case this function is basically the reciprocal
of a rising factorial, or where both arguments are smallish integers: in
which case table lookup of factorials can be used to calculate the ratio.